1. Field of the Invention
This invention relates to filters for use in video applications, and more particularly to a filter that reduces noise contained in digital video signals.
2. Discussion of the Related Art
It is known that, in general, a reduction of the noise contained in signals can be obtained through filters of the low-pass type having a predetermined cutoff frequency. However, the use of such filters involves in most cases a reduction of a signal portion which may contain useful information.
With video signals, for example, the use of low-pass filters involves an attenuation of the high-frequency components which contain useful information about the image outline.
To alleviate this drawback, adaptive filters have been employed which adapt their frequency response to the characteristics of the signals that appear at their inputs.
Such filters require, however, considerable computation work for their implementation due to their highly complicated circuitry. In video signal processing, noise reduction has been achieved using non linear filters including a filter known in literature as a median filter, as well as linear filters of the bi-dimensional low-pass type formed by conventional synthesis methods which often result in loss of useful information.
For video signals, filters have also been used which operate in a logic mode based on fuzzy rules.
Fuzzy methods for use in filtering techniques directed to reduce the noise contained in video signals are described, for example, in the following articles: F. Russo and G. Ramponi, "Fuzzy Operators for Sharpening of Noisy Images", Electronics Letters, Vol. 28, No. 18, August 1992; F. Russo and G. Ramponi, "Working on Image Data Using Fuzzy Rules", Signal Processing IV Theories and Applications, Proceedings of EUSIPCO-92, Brussels, Belgium, August 1992 and C. W. Tao and W. E. Thompson, "A Fuzzy If-Then Approach to Edge Detection", Proceedings of Second IEEE Conference on Fuzzy System, San Francisco, Calif., March/April 1993. Filters operating on fuzzy logic are, however, complex architectures at the structural level and costly to manufacture.